Method for forming an orientation film of photopolymer in a liquid crystal display

ABSTRACT

A method for the formation of orientation film of a liquid crystal display. 
     The method comprises the steps of forming polyvinyl-4-fluorocinnamate (hereinafter &#34;PVCN-F&#34;) film on the surfaces of two opposite substrates and irradiating the two PVCN-F films formed with linearly polarized UV lights having different energy. 
     The method in accordance with the present invention is capable of not only providing the pretilt angle to a fabricated LCD but also adjusting it by illuminating two substrates with linearly polarized UV beams having different energies from each other, respectively. In addition, the LCD fabricated by the method according to the present invention requires a much lower driving voltage, as compared with the conventional LCD having an orientation film of planar structure. Furthermore, phase distortion and light scattering phenomena do not occur in the LCD, so that display characteristics such as contrast and the like can be improved. 
     The orientation film is formed of polyvinyl-4-fluorocinnamate which is highly resistant to heat, so that the LCD according to the present invention has an excellent thermostability.

BACKGROUND OF THE INVENTION

The present invention relates in general to a method for the formationof orientation film of a liquid crystal display (hereinafter "LCD"), andmore particularly to a method for forming an orientation film ofphotopolymer in an LCD, capable of providing a pretilt angle to theorientation film as well as adjusting it.

In a typical electrically switched LCD, a liquid crystal material issealed between two glass or plastic plates that are provided withtransparent conducting electrodes.

Hereinafter, a conventional LCD structure and method for fabricating thesame will be discussed along with the problems generated therein withrespect to the pretilt angle for better understanding of the backgroundof the invention, referring to FIG. 1. There is a structure for aconventional LCD. As shown in FIG. 1, the inner surfaces of transparentglass substrates 1 and 1A are coated with transparent conductingelectrodes 2 and 2A and are then covered with orientation films 3 and3A, respectively. The inner surfaces are adhered to each other with asealing material (not shown) so as to form a space, into which a liquidcrystal material 4 is injected, the other surfaces of the glasssubstrates 1 and 1A being provided with polarizers 5 and 5A.

In such an LCD, it is required to align the liquid crystal injectedbetween the two glass substrates in a uniform direction for a constantbrightness and a high contrast ratio.

Several techniques for aligning the liquid crystal in a uniformdirection are known. For example, there has been proposed such a methodthat an orientation film is deposited over a substrate which issubsequently subjected to the mechanical treatment of rubbing to formmicrogrooves, so that liquid crystal molecules may be aligned regularlyon the entire surface of the orientation film therethrough. In thistechnique, a polyimide-type or polyamide-type polymeric material iswidely used as a material for the orientation film. However, thistechnique employing the mechanical rubbing is disadvantageous in thatthe produced micro grooves inherently have defects, which cause randomphase distortion and light scattering phenomena, thereby adverselyaffecting the operational characteristics of the display. In addition,static electricity generated during the mechanical rubbing of thepolymer surface is believed to cause defects in active matrix displays.Furthermore, it is practically impossible to locally orient selectedregions of the surface with the orientation of each different region.

In order to overcome the aforementioned disadvantages, a prepolymer,photosensitive material, has been proposed, which is polymerized bylight to form the orientation film. The prepolymer includespolyvinyl-4-methoxycinnamic acid (hereinafter "PVCN-M") which isprepared by the reaction of polyvinyl alcohol (hereinafter "PVA") with4-methoxy cinnamic acid. According to this technique, irradiation oflinearly polarized ultraviolet (hereinafter "UV")light makes PVCN-Mcrosslinked, so that an oriented film of net structure is produced. Thisphotopolymeric orientation film allows the liquid crystal molecules tobe oriented in a preferred direction of planar structure. However, thereexist problems in the orientation film formed by the photopolymerizationof PVCN-M. For example, since the orientation film of PVCN-M has a badthermostability, as ambient temperature becomes more than approximately50° C., distortion emerges therein. In addition, the orientation filmprovides zero pretilt angle. As a result of zero pretilt, theelectro-optical characteristics of the LCD based on the photosensitivematerial are inferior to those based on the rubbed polymer. For example,higher driving voltages and worse contrast are shown.

SUMMARY OF THE INVENTION

For solving the aforementioned problems, a need exists for a method forforming an orientation film, which is highly resistant to heat and whichis capable of providing a suitable pretilt angle to an LCD.

Accordingly, in an aspect of the present invention, there is provided amethod for forming a photopolymeric orientation film in an LCD, wherebythe film can be highly resistant to heat.

According to another aspect of the present invention, there is provideda method for forming a photopolymeric orientation film in an LCD,whereby a pretilt angle can be so formed as to have a free value to someextent in the LCD.

According to a further aspect of the invention, there is provided amethod for forming a photopolymeric orientation film in an LCD, wherebythe LCD is improved greatly in phase distortion and light scatteringphenomena.

The above objects are accomplished by providing a method which comprisesthe steps of forming polyvinyl-4-fluorocinnamate (hereinafter "PVCN-F")film on the surfaces of two opposite substrates and irradiating the twoPVCN-F films formed with linearly polarized UV lights having a differentenergy for each film.

The above and other objects and advantages of the present invention willbecome more apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described in thespecification and particularly pointed out in claims, the followingdescription and the annexed drawing setting forth in detail a certainillustrative embodiment of the invention, this being indicative,however, of but one of the various ways in which the principles of theinvention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic sectional elevation view showing a conventionalLCD structure;

FIG. 2 is a schematic illustration of an experimental setup for theillumination of linearly polarized UV light and for the measurement ofbirefringence;

FIG. 3 is a schematic illustration of an experimental setup for pretiltangle measurements;

FIG. 4 is a curve illustrating the relation between an inducedbirefringence in a PVCN-F film and a UV illumination time, according tothe invention; and

FIG. 5 is a plot illustrating the relation between a pretilt angle andan illumination time difference on the basis of the results of examplesaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the invention will be described indetail.

The PVCN-F, used as a material for the photopolymeric orientation film,is prepared by reacting PVA with fluorocinnamic acid derivatives,wherein fluorine is substituted on a benzene ring of cinnamoyl molecule.It is necessary to dissolve PVCN-F in a solvent in order to depositPVCN-F over the surface of glass substrate on which a transparentelectrode and a thin film transistor are formed. For this, PVCN-F isdissolved in a mixture of 1,2-dichloroethane (hereinafter "DCE") andchlorobenzene (hereinafter "CB"). If a PVCN-F solution of low molecularweight is obtained, the weight ratio of DCE and CB may be 1:1. In themeanwhile, for preparing a solution of high molecular weight, the weightratio of DCE and CB may be 1:4. Since the concentration of the PVCN-Fsolution is dependent on the thickness of film which is to be coated,the concentration is determined by the film thickness. For example, ifthe film is to be coated in a thickness of approximately 500 nm, aPVCN-F solution with a concentration of 4 g/l may be used, in which 4 gof PVCN-F is dissolved in 1 1 of a 1:4 mixture of DCE and CB.

After preparing the solution of PVCN-F in a mixture of DCE and CB, thesolution is added dropwise with a pipette on the central portion of theglass substrate on which a transparent electrode and a thin filmtransistor are formed. Subsequently, an orientation film is formed onthe substrate, using a spin coating instrument. The spin coating processis carried out for 20 to 30 seconds in a rotational speed of 3 to 5×10³rpm. Following the spin coating, the resulting substrate is subjected tothe treatment of prebaking at approximately 50° C. for approximately 30minutes.

In accordance with the present invention, illumination of linearlypolarized UV light causes the deposited polymer to be crosslinked,providing a pretilt angle thereto, as described later. The pretilt angleformed in an LCD may be adjusted by irradiating two films deposited onglass substrates of the LCD with two UV lights of different energy,respectively.

The two opposite glass substrates are attached to each other in aconventional method, and then a liquid crystal material is injectedtherebetween, utilizing capillary action.

The detailed description will be given next for the film illuminationand birefringence measurement with reference to FIG. 2. There is aschematic view illustrating an experimental setup for illuminatinglinearly polarized UV light to the deposited prepolymer material and formeasuring birefringence. As shown in FIG. 2, UV light generated from amercury lamp 11 with an average power of approximately 250 to 500 W islinearly polarized through a lens 12 and a prism 13. The linearlypolarized UV light obtained, enters a splitting mirror 14 which passesone direction of the light therethrough and reflects other directions,so that the passed light illuminates the photosensitive prepolymermaterial (PVCN-F) 15 coated on a glass substrate 16. As a result ofillumination, the initially isotropic polymer becomes an anisotropicpolymer.

An induced birefringence can be detected by a measuring system includingthe splitting mirror 14, polarizers 17 and 17A, a helium-neon laser 18,a photodiode 19 and an oscillograph 20. The illumination state can bemonitored by measuring the birefringence since the anisotropy isrepresented as birefringence.

Turning now to FIG. 3, there is schematic view illustrating anexperimental setup for pretilt angle measurements. As shown in FIG. 3, abeam generated from helium-neon laser 21 is incident on an LCD cell 24rotating between polarizers 23 and 23A which are transverse to the pathof the beam. The cell is adjusted on a table equipped with a step motor26. The intensity of the beam I passed through the diaphragm 22 isrecorded as a function of the angle between the axis normal to the celland wave vector K of a laser beam. As the beam further passes throughanother diaphragm 22A and enters a light-receiving device, photodiode25, the value of intensity is displayed in a monitor 27 provided to acomputer which receives a signal relating to information for theintensity through the photodiode 25. The pretilt angle was determined bythe position of symmetry axis of the τ(O) curve, represented as thefollowing formula: θ≈O_(s) /(n_(e) +n_(o)), wherein O_(s) is an anglecorresponding to the symmetry axis, and n_(e) and n_(o) are the LCrefractive indices for extraordinary and ordinary waves, respectively.

Referring now to FIG. 4, there is a graph illustrating the relationbetween the birefringence of the photopolymer and the illumination time,according to the present invention. From the drawing, for example it isapparent that when the intensity of linearly polarized UV beam isconstant, the birefringence is almost proportional to the illuminationtime until 40 minutes.

EXAMPLE 1

A first PVCN-F film coated on a glass substrate was placed in theinstrument shown in FIG. 1, and was exposed to linearly polarized UVlight having an intensity of approximately 25 mW for 10 minutes. Theilluminated area was approximately 1×1.5 cm². A second PVCN-F film onanother glass substrate was also exposed to the same beam for 20 minutes(illumination time difference Δt_(ex) =10 minutes).

The two resulting glass substrates were attached to each other and aliquid crystal material was injected in a conventional manner tofabricate an LCD. Using the optical equipment shown in FIG. 2, thepretilt angle of the fabricated LCD was measured, as explained above.

The result is given as shown in the following Table 1 and is plotted inFIG. 5.

EXAMPLE 2

Two PVCN-F films were exposed in a manner similar to that of Example 1except that the illumination times for the two films was 10 minutes,respectively (illumination time difference Δt_(ex) =0 minute). Theresulting glass substrates were attached to each other, followed by theinjection of liquid crystal therebetween, so as to fabricate an LCD. Thepretilt angle of the fabricated LCD was measured.

The result is given as shown in the following Table 1 and is plotted inFIG. 5.

EXAMPLE 3

An LCD was prepared in a manner similar to that of Example 1 except thatthe first PVCN-F polymer was exposed for 10 minutes and the secondPVCN-F polymer was exposed for 30 minutes (illumination time differenceΔt_(ex) =20 minutes).

The pretilt angle of the fabricated LCD was measured.

The result is given as shown in the following Table 1 and is plotted inFIG. 5.

EXAMPLE 4

An LCD was prepared in a manner similar to that of Example 1 except thatthe illumination times for the first and the second PVCN-F polymers were90 and 150 minutes, respectively (illumination time difference Δt_(ex)=60 minutes).

The pretilt angle of the prepared LCD was measured.

The result is given as shown in the following Table 1 and is plotted inFIG. 5. cl EXAMPLE 5

An LCD was fabricated in a manner similar to that of Example 1 exceptthat the illumination times for the first and the second PVCN-F polymerswere 10 and 90 minutes, respectively (illumination time differenceΔt_(ex) =80 minutes).

The pretilt angle of the fabricated LCD was measured.

The result is given as shown in the following Table 1 and is plotted inFIG. 5.

EXAMPLE 6

An LCD was fabricated in a manner similar to that of Example 1 exceptthat the first and the second PVCN-F polymers were exposed for 10 and 70minutes, respectively (illumination time difference Δt_(ex) =60minutes).

The pretilt angle of the fabricated LCD was measured.

The result is given as shown in the following Table 1 and is plotted inFIG. 5.

                  TABLE 1                                                         ______________________________________                                        Example Pretilt   Illum. Time (min)                                                                          Time                                           No.     Angle (°)                                                                        First   Second Difference (min)                             ______________________________________                                        1       10        10      20     10                                           2        0        10      10      0                                           3         13.5    10      30     20                                           4        0        90      150    60                                           5       18        10      90     80                                           ______________________________________                                    

Referring to FIG. 5, there is illustrated the relation between thepretilt angle and the time difference illuminating the first and secondsubstrates with the light. As shown in the figure, the pretilt angle isproportional to the illumination time difference and the gradient islarge within 30 minutes of the time difference whereas it is small whenthe difference is greater than 30 minutes.

From the Examples, it is recognized that the pretilt angle θ becomesgreater as the time difference is larger under the constant intensity oflight. However, this is effective within 90 minutes of the illuminationtime. In other words, when the film is exposed for a time longer than 90minutes, the pretilt angle is not obtained even though the timedifference is very large.

In the meanwhile, it should be noted that similar or the same results asillustrated above are obtained when the intensities of linearlypolarized incident UV light are different from one another under aconstant illumination time. That is, since, as proposed in presentinvention, the pretilt angle is dependent on the difference of theillumination energy which is, in turn, proportional to both theintensity of light and the illumination time (intensity×time). When theillumination time is the same, the stronger the light intensity of theincident beam above the critical intensity the greater the pretiltangle.

As explained above, the method in accordance with the present inventionis capable of not only providing the pretilt angle to a fabricated LCDbut also adjusting it by illuminating substrates with two, linearlypolarized UV beams having different energies each other, respectively.In addition, the LCD fabricated by the method according to the presentinvention requires a greatly low driving voltage, as compared with theLCD using PVCN-M as an orientation material and having an orientationfilm of planar structure. Furthermore, phase distortion and lightscattering phenomena do not occur in the LCD, so that displaycharacteristics such as contrast and the like can be improved.

Whilst the present invention has been described with reference tocertain preferred embodiments and examples, it will be appreciated bythose skilled in the art that numerous variations and modifications arepossible without departing from the spirit or scope of the invention asbroadly described.

What is claimed is:
 1. A method for forming orientation films for aliquid crystal display, comprising the steps of:forming a film ofpolyvinyl-4-fluorocinnamate polymer on the surface of a first substrate;forming a film of polyvinyl-4-fluorocinnamate polymer on the surface ofa second substrate; and irradiating the films with different amounts oflinearly polarized ultraviolet light such that the resulting liquidcrystal display has a pretilt angle.
 2. A method according to claim 1,wherein each of the steps of forming a film ofpolyvinyl-4-fluorocinnamate polymer comprises dissolving saidpolyvinyl-4-fluorocinnamate polymer in a mixture consisting of1,2-dichloroethane and chlorobenzene in a definite ratio and using aspin coating instrument.
 3. A method according to claim 1, wherein thetwo films are irradiated for different times, with a constant intensityof the linearly polarized ultra violet light.
 4. A method according toclaim 1, wherein the two films are irradiated with linearly polarizedultra violet light of different intensities, with the illumination timeconstant.